In one instance, a person may need respiratory assistance as a result of disease and injuries of various kinds. The respiratory assistance can encompass everything from facilitating spontaneous breathing to full-time respiratory pacing. Typically, a mechanical ventilator is employed to provide the needed level of respiratory assistance.
Studies show that long-term respiratory support leads to diaphragm muscle weakness. The diaphragm muscle is largely responsible for a person's ability to inspire and will begin to atrophy as soon as 18 hours following continuous ventilator support. The severity of atrophy is exacerbated over time. In many instances, the atrophy is so severe that the person loses the ability to breathe spontaneously upon removal of the respirator.
When persons cannot breathe reliably on their own, they must undergo a weaning process designed to free them from the respirator. The weaning process may last days, weeks or months, and is dependent on the severity of the atrophy. Weaning over a long term increases a person's discomfort level and risk of developing a secondary disease (e.g., pneumonia).
The time-course and various challenges of weaning are important to the caregiver and health care provider. Between 1.2 and 1.8 million persons fail at least 1 weaning attempt per year. Long-term weaning can contribute to a loss of life. Furthermore, about 6 million people are mechanically ventilated each year at a cost of approximately $1,500.00 per patient per day. The weaning period accounts for about 42% of the time people are mechanically ventilated. This adds to the overall cost of medical care.
Diaphragm atrophy is a recognized issue in persons with spinal cord injuries, and it is reversible. Electrical stimulation of the phrenic nerves via implanted electrodes is used to strengthen the weakened diaphragm muscle in spinal cord injured persons who are preparing for full-time respiratory pacing. The conditioning period is variable, ranging from 3 to 16 months, and is dependent on the severity of the atrophy.
The invasive method of causing the diaphragm muscle to contract carries with it a risk of infection, a need for surgery and inpatient stays, discomfort due to having electrodes implanted into the body, and higher costs for the person to bear. Thus, non-invasive methods of causing contraction of the diaphragm muscle are preferred for any patient undergoing a weaning or conditioning process. Building on the success of using implanted devices to condition atrophied diaphragm muscles, non-invasive electric methods have been explored.
It has been shown that transcutaneous, electrical-stimulation of the human neck region can activate the phrenic nerves and drive contraction of the diaphragm muscle. Studies have elicited diaphragm muscle contraction with surface electrical stimulation to investigate breathing muscle atrophy. Specifically, the application of a single pulse of electrical stimulation (e.g., >1 ms pulse duration) to the neck region where the phrenic nerve is located can elicit maximal diaphragmatic pressures in humans. Shorter pulse durations (100 μs) of monophasic or biphasic constant-voltage square-wave pulses with large stimulation amplitudes (approaching 300 V) can also be used to achieve maximal diaphragm muscle contraction.
Despite these accomplishments, several problems prevent these methods from being successful. One problem is the activation of pain receptors in the proximity of the electrodes (i.e., neck). This pain sensation is reported to be quite significant, and is caused by activation of nociceptors that are located in the skin beneath the electrodes. Painful treatment methods can cause a patient to undergo extreme stress in anticipation of and during treatment. Another problem is the difficulty in placing the electrode at the appropriate area of the neck. If initially the electrode is not placed correctly, the diaphragm will not contract and unwanted muscle contractions from superficial musculature will result. This will require a painful search for the ideal stimulation site.
As such, there remains a need for a stimulation system and method that activates the diaphragm muscle in humans by stimulating the phrenic nerve in a reliable, non-invasive manner. There is a further need to stimulate the phrenic nerve without recruiting somatic pain receptors and without eliciting extraneous muscle contractions. There is another need to provide stimulation to the phrenic nerve in a cost-effective manner. In addition, there is a need for a phrenic nerve stimulation system and method that is widely accessible to clinicians.
In another instance, persons with obstructive sleep apnea need assistance with breathing during periods of sleep. The most common method of treating sleep apnea is with the use of a device that applies continuous positive airway pressure. However, this requires that the person wear a face mask or the like, which for some persons is unacceptable. Thus, there remains a need for a stimulation system and method that causes breathing in humans by stimulating the hypoglossal nerve in a reliable, non-invasive manner.
In another instance, persons suffering from bronchoconstriction (i.e., asthma; COPD) or headache conditions (i.e., migraine; cluster migraine) need adequate acute and/or prophylactic treatment options. Initial studies suggest that vagal nerve stimulation can mitigate or abolish asthma, COPD and a variety of headache types. Therefore, there is a need for a stimulation system and method to activate the vagus nerve non-invasively, without co-activating the surrounding sensitive structures.